As part of radio resource management, a mobile user terminal (user equipment, UE) is moved from one resource state to another in order to maximise the efficiency of the available radio resources (e.g. power, bandwidth). The Universal Mobile Telecommunications System (UMTS) standard requires each mobile user terminal to take either of two states for active data transmission, namely cell forward access channel (Cell-FACH) state and cell dedicated channel (Cell-DCH) state, (see for example Third Generation Partnership Project (3GPP) Technical Specification 25.331).
In Cell-FACH state, a shared channel, specifically the forward access channel (FACH), is used shared between a group of mobile user terminals and having no associated dedicated physical control channel. The forward access channel (FACH) is used for low volume data and is similar to the broadcast channel and random access channel of older mobile technologies. It may typically be used e.g. for the viewing time after a Web page download. Use of the forward access channel (FACH) gives improved cell capacity at low data rates and thus improves the battery life of the mobile user terminal due to the lower transmission power that is required.
On the other hand, Cell-DCH state uses a dedicated channel (DCH) to a mobile user terminal which has a dedicated physical control channel, including transmit power control information and pilot information, continuously transmitted. A dedicated channel (DCH) of a cell is typically used for high volume data or voice.
Whilst in Cell-DCH state, the mobile user terminal reports to the base station when criteria are met such as when the measured signal from a monitored base station goes above or below a threshold, whereupon the base station may choose to add or remove a soft handover leg (i.e soft handover connection, also known as soft handoff connection).
The standards provide a variety of measurement reporting methods for reporting handover measurements (3GPP Technical Specification 25.331 Section 14.1.2), but whilst the mobile user terminal is in Cell-FACH state, measurements on received signals are disabled as they might result in frequent measurement reports back to the base station thereby reducing the capacity of the channel for user data or voice.
Furthermore, the standards provide no reliable means for determining the transmit power at which each Forward Access Channel (FACH) frame should be broadcast to the mobile user terminal (UE), so the FACH channel is always broadcast at a high power sufficient to cover the whole cell.
Furthermore, as regards criteria for transiting between Cell-FACH state and Cell-DCH state, a known criterion is level of occupancy of the buffer (in the mobile user terminal) which stores data to be sent. A buffer occupancy measurement is passed from the mobile user terminal to the base station where a decision to instruct a change of state is made.
As further background, Dynamic Bearer Control (DBC) is a commonly understood mechanism defined within the 3GPP standards which determines whether, and to what extent, to grant resources, i.e to allocate a data rate, based upon the estimated power consumption. The DBC functional block usually receives no power information or soft handover information. Accordingly, the DBC functional block assumes that a mobile user terminal requires maximum power when transiting into a Cell-DCH state so may allocate a lower data rate than it needs to, or even refuse to provide DCH resources.
When the base station decides that the mobile user terminal in the Cell-FACH state should transition to Cell-DCH state, it must do so without any soft handover legs i.e. without second or further radio connections between the mobile user terminal and other cells than the primary one. This is because there are no soft handover connections to mobile user terminals in the Cell-FACH state. Also the base station and mobile user terminal must start with a high initial transmit power in order to ensure a rapid synchronisation between the mobile user terminal and the base station. These result in a reduction in the traffic capacity, particularly in cases where the transitions between the two states are frequent.